Simplify F-Curves of Shape Key animation
The simplify f-curves operator has been created in a rigid way that only looks at the objects animation data. Not all animation data is stored in the one place, for example the shape key animation is stored as part of the mesh data, not as part of the object animation.

While some more work should be done to make the simplify f-curves a little more flexible, this modified version(v1.0.4) allows the shape key f-curves to be simplified. You can download and unzip this version, then use the install addon from file button to select the unzipped py file, blender should then replace the existing addon with this version, if you have permissions to write over the file.